A role for nitric oxide in transmission-modulating immunity to Malaria

Malaria is endemic to some 90 countries worldwide, causing clinical disease in up to 500 million people annually, of whom between 1.5-2.7 million die. The asexual erythrocytic stages of Plasmodium have been explored in depth in an effort to develop effective anti-disease control measures for human malaria. However, despite the lack of a definitive asexual stage vaccine, only recently have the sexual stages of Plasmodium undergone similar scrutiny. It is these stages that infect the mosquito and hence strategies designed to ablate sexual stage development are termed transmission-blocking. Since the discovery that mammalian cells produce nitric oxide (NO), considerable attention has been focused on its role in the cell-mediated killing of microbes. While vertebrate-borne NO and other reactive nitrogen intermediates (RNI) are relevant in the context of blood-borne malaria, they must also be considered in examining the immunology of sexual stage malaria, both within the vertebrate and following ingestion by the mosquito; the blood meal will contain not only red blood cells (RBC) but also a range of leukocytes and their associated chemical mediators. Our previous observations suggest that vertebrate-derived NO may be a concentration-dependent modulator of exflagellation such that levels that are constitutively expressed (0.5 μM) are able to enhance exflagellation whereas those produced inducibly by immune-activated macrophages (50-150 μM), via inducible nitric oxide synthase (iNOS), modulate the process negatively reaching complete abrogation. In addition, it has recently been demonstrated that Anopheles mosquitoes are able to synthesise a homologue of iNOS (termed AsNOS) in response to microbial infection. It also appears that increased NO synthesis in P. falciparum malaria can be directly elicited by soluble factors released by the blood stage parasite itself. It is the interactions between these three putative sources of RNI within the invertebrate vector that require elucidation if cell-mediated immune responses to Plasmodium inside the mosquito are to be better understood. Here, using the P. berghei murine malaria model, we demonstrate that vertebrate-derived NO modulates both the synthesis of AsNOS by Anopheles stephensi, and the infectivity of parasites to An. stephensi (at the oocyst and sporozoite stages) in a concentration-dependent manner.